Gas insulated transformers



June 28, 1960 TsENG w. LlAo ETAL 2,943,134

GAS INSULATED TRANSFORMERS 2 Sheets-Sheet 1 Filed Jan. 25. 1955 5L Ez Maffe/vnr: M5

/17 renr Tseng W. L/ao,

June 28, 1960 TsENG w. LlAo ETAL 2,943,134

GAS INSULATED TRANsFoRMERs 2 Sheets-Sheet 2 Filed Jan. 25, 1955 HHH M5 wm ma T510 United States Patent O GAS INSULATED TRANSFORMERS Tseng W. Liao, Pittslield, Mass., and Clyde B. Musick, Rome, Ga., assignors to General Electric Company, a corporation of New York Filed Jan. 2s, 195s, ser. No. 4ss,9zs 7 claims. (C1. 174-16) category. These electronegative gases, of which the iluorogases SP6 and C4F10 are examples, are said to have an elect-ron affinity, or in other words, theyl exhibit an apparent electroncatching property forming negative ions. While the reasons for this property are not fully understood, it is thought to b e due to an inherent tendency of elements to iill out incomplete electron orbits. .The halogens need only one electron to lill out their 1 incomplete orbits, and it has been found that the halogens exhibit great electron ailinity.

Electronegative gases have been used, either alone orv .with various inert gases such as nitrogen, as insulation in electrical apparatus. It is well known that the dielectrically weakest region in electrical apparatus is the region of maximum electric stress. Thus in a transformer Vthe weakest region is at the end turns of the transformer windings, where the electrical tield is concentrated at the corners. The conventional manner of producing a more uniform field lat the end turns of such windings is to round off :the conductor corners. From an electrical standpoint such rounding ofi may produce the optimum conditions, but, due to the large amount of space required for efficiently rounding the end turns of the windings,

` only a small advantage may be derived practically from this method. It was found that the round off eliect can be accomplished by an artificial formationl of a cloud of negative ions in the vicinity of the winding corner. fIt is apparent that when the winding corner is of negative polarity, the negative ion cloud acts like a shield minimizing the non-uniformity of electric field at the winding corner. Although this cloud should also reduce the effective spacing between the corner and the opposite v winding, yet the decrease of electrical stress due to the rounding effect over-balances the apparent decrease in spacing; thus resulting in less danger of breakdown from the winding corner.

When the winding corner becomes electrically positive,- then the cloud` of negative ions adheresI closely to -the positive winding, resulting in a reduction of the net electric stress in the remaining space between the windings. It is also conceivable that the cloud of negative ions continuingly neutralizesA any positive ions that might be produced during the necessary process of breakdown from the winding corner'of positive polarity.

From the standpoint of obtaining the best insulation properties in electrical equipment, it is desirable to use as high al concentration of electronegative gas as possible. The greater the concentration of such gases, the more effectivev is the negative ion cloud in increasing the dielectric strength of the insulating medium. One disadvantage of the higher concentrations of electronegative gas,

"however, is the'highfrelative cost of the gas.

2,943,134 Patented June 28, 1960 ice s Another aspect is in the consideration of condensation. When gas insulation is used in certain electrical apparatus, such as outdoor transformers, it is essential that the gas insulation have a low condensation temperature so that it will not condense at the lower operating temperature and lose its effectiveness as a dielectric medium. This requirement is especially true when the gas is under pressure. To attain low condensation temperature gas mixtures can be used. vAlthough it is desirable to have as high a ratio of electronegative gas to inert gas of Vlow condensation point as possible in order to achieve a given breakdown strength requirement, the greater percentage of electronegative gas present results in an' increase in the condensation point` of the mixture. For instance, a one to one ratio mixture of C4111., and nitrogen condenses at -21 C. under atmospheric pressure, a one to ve ratio mixture condenses at -35' C. under atmospheric pressure, and a one to ten ratio mixture condenses at C. under atmospheric pressure.

Thus, while the breakdown strength ofa gaseous insulating medium is increased by employing an electronegative gas, the cost is increased, and the condensation point is also increased.

Briey stated, the present invention improves the insulation characteristics of gaseous insulation containing electronegative gases by providing means for the introduction of additional negative ions into the gas. In the past the only probable source of negative ionsV for the gas was from the relatively sharp corners at the ends of the winding turns. In accordance with the principles of this invention, it is now believed that an insufficient quantity of charged particles arc emitted from Vthese regions to obtain the optimum insulation characteristics in the gas and that additional sources of negative ions are necessary. Contrary to previous practices and beliefs, the insulation characteristics may be improved by providinga plurality of sharp points or corners in regions of high electric stress, since these points provide additionalnegative ions for the gas. Other means of providing additional ions for the gas are also contemplated by this invention, such as by subjecting the gas to bombardment from a nuclearradiation source. The source may be in the form of an insulating radio-active tape or foil around the conductors, or may be a separate device located in the gas circulation system. y.

By thus providing additional charges in the gas, -a

i lower ratio of electronegative gas to inert gas may be used for the same breakdown strength. This results in a lower cost for the gas mixture, and also a lower gas condensation temperature. y I

It is therefore an object of this invention to provide means for increasing the breakdown strength of thegas mixture in gaseous insulated electric apparatus when at least a portion of the gas mixture is anelectronegatve gas.

Another object is to provide means for reducing the electronegative gas to inert, gas ratio required in gaseous insulated apparatus for a given dielectric `breakdown strength. v

Still another object of this invention isto provide means for emitting additional charged particles into an electronegative gas to increase the insulation characteristics thereof.

A further object is to provide a plurality of relatively sharppointsl or corners at regions of high electricstress in electronegative gas insulated electric apparatus to imp rove the insulation characteristics of the gaseous insulation. 4

A still further object of this invention is to provide a source of radiation in velectronegative gas -insulated electric apparatus to improve the insulating characteristics of the gaseous insulation. Y

Our invention will be better understood from the fol t theA gas mixture.

Villustrating one embodiment of this invention as applied A to the windings of the transformer of Fig. 1,

,Fig.l3 is a partially cross sectional perspective view illustrating a second embodiment of this invention as applied tothe windings of the transformer of Fig. l, and

Fig. 4` is a view of atransformer illustrating althird embodiment of this inventionas applied to gas insulated transformers;

Referring now to Fig. 1 of the drawing, a gas insulated transformer is therein illustrated having a magnetic core 11 about which is mounted an electrical winding assembly '12'. The winding assembly is comprised of an inner winding cylinder 13, a low voltage Winding '14, an insulation barrier 15 around the low voltage winding, and a high voltage winding 16 wound over the insulation barrier 15. A plurality of spaces 17 are provided between the windings and insulation for circulation of a gaseous insulation 18 around the windings. 'I'he gas is generally a mixture of an electronegative gas such as SFG or', C 4F10 with an inert gas such as nitrogen. The

flow voltage windings 14 extend beyond the high voltage Vwindings 16, and a plurality of insulating spacers 19 extending to end turns of the high voltage windings are provided to aid in the support of the coils. The entire `assembly is enclosed in an air tight tank 20, and a plurality of conductors 21 connected to the windings feed through bushings 22 in the tank wall.

In a transformer having the construction of Fig. l, the highest electrical stresses occur in the non-linear ren gion surrounding the corner 23 of the high voltage winding closest to the low voltage winding. Investigations of the effect produced at the corner 23 or electrically similar constructions, on electronegative gas insulation indi- `cate that a negative ion cloud surrounds the corner 23. This cloud is apparently due to the electron afiinity of the vgas, whereby electrons emitted from the corner are caught by the gas. It has now been found that the insulation properties of the gas surrounding such a corner may be improved by providing means for producing additional negative ions in the gas.

In the embodiment of this invention illustrated in Fig. 2, additional charges are introduced into the electronegative gas by providing a plurality of relatively sharp points around the corner 23. In the illustration, metallic fibers 30 suchas steel wool fibers are affixed to a varnish coating on the winding insulation 31. Ordinarily, it would be expected that such a practice would increase the danger of breakdown from the corner 23 due to the .resultant increase in electrical stresses around the sharp points. It has been found, however, that the presence of si.ve5 steel wool is equivalent toboosting the percent content of the comparatively high cost electronegative, gas in Other material` consisting ofV a large number; 0f sharp corners such as points or-ridges would 'I give. similar results, and these pointsvor ridges may be in contact with the conductor or formedl on, theA conductor if desired.

Referring now to Fig. 3, the same effect of increasing the insulation characteristics. is produced by wrapping the end turn 40 of the high voltage winding 16 with a radioactive tape or foil 41 in order to bombard the electronegative gas with additional charged particles, and thereby produce the effect of adding more electronegative gas to the gas mixture. It is preferred that the radioactive tape be an alpha particle emitter such as polonium, since alpha particles have the greatest effect in the desired region due to their short'paths of travel. Commercially available radioactive polonium foils have been found suitable for this purpose, however, anyv material that produces a suflicient number of negative ions in the gaseous insulation may be used. i

Referring now to Fig. 4, in a third embodiment of this invention, a transformer having a core 50' and windings 51 is enclosed in an airtight tank 52. The'gaseous insulation is circulated through an outlet tube 53, an'upper yheader 54, a plurality of external ycooling tubes 55, a

lower header 56, and returns to the tank through a blower 57. The core and winding construction may be the same as illustrated in Fig. l, whereby the gas circulates through the windings. Since the effect of the electronegative gas insulation appears to be dependent upon the presence of negatively charged particles in the gas, a radiation source 58 is positioned in the circulation system. As in the case of the radioactive tape shown in Fig. 3, it is preferred that the source emit primarily alpha particles, although anyd source that forms negative ions in the gas may be use l Various other means for introducing negatively charged particles in the gas may also be employed without departing from the spirit of this invention, and any means by which the gas is ionized to produce these results is contemplated. For example, a separate corona discharge source, a thermal ionizing source, or an electromagnetic source may be used to ionize the gas. t

From the foregoing description it is seen that the effective insulation properties of an electronegative gas containing gaseous insulation are greatly improved by providing means for producing additional negative charges in the gas. The charges may be introduced into the gas either at the point of highest electrical stress, or elsewhere in the gas circulation system. Although the invention has been described with particular reference to transformers, it is readily adaptable to other gas insulated electrical apparatus.

It will be understood, of course, that, while the forms of the invention herein shown and described constitute preferred embodiments of the invention, itis not intended herein to illustrate all of the possible equivalent forms or ramifications of the invention. It will also beunderstood that the words used are words of' description rather than of limitation, and that various changes may be made-without departing from the spirit or scope of the invention herein disclosed.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical apparatus enclosed in a gas tight chamber, said electrical apparatus having spaced apart conducting means which are at different electrical potentials during use of said apparatus, gaseous insulation comprised at least in part of an electronegative gas within said chamber and between said conducting means, solid insulation wrapped around said conducting means, and means for introducing negative ions in said gaseous insulation in addition to ions that are normallypresent therein during use of said apparatus, said last mentioned means comprising a plurality of relatively sharp metallic points disposed on the surface of the solid insulation wrapping 'of at least one of said conducting means in a region of high electric stress, thus increasing the dielectric strengthf of said gaseous insulation.

Z. An electrical apparatus enclosed ina gas. tight chamber, said electrical apparatushavingspaced apartconduing means which are at different electrical potentials during use of said apparatus, gaseous insulation comprised at least in part of an electronegative gas within said chamber and between said conducting means, solid insulation wrapped around said conducting means, and a plurality of metallic bers disposed on the surface of the solid insulation wrapping of at least one of said conducting means in a region of high electric stress, thus increasing the dielectric strength of said gaseous insulation.

3. The electrical apparatus of claim 2 wherein said metallic bers are steel wool fibers, and said steel wool fibers are imbedded in a varnish coating on said solid in'- sulation.

4. An electrical apparatus enclosed in a gas tight chamber, said electrical apparatus having spaced apart conducting means which are at different electrical potentials during use of said apparatus, gaseous insulation comprised at least in part of an electronegative gas within said cham ber and between said conducting means, and means for introducing negative ions in said gaseous insulation in addition to ions that are normally present therein during use of said apparatus, said last mentioned means comprising a nuclear radiation emitting tape wrapped around at least one of said conducting means in a region of high electric stress, thus increasing the dielectric strength of said gaseous insulation.

5. An electrical apparatus enclosed in a gas tight chamber, said electrical apparatus having spaced apart conducting means which are at diiferent electric potentials during use of said apparatus, gaseous insulation comprised at least in part of an electronegative gas within said conducting means and an alpha particle emitting radioactive tape wrapped around at least one of said conducting means in a region of high electric stress, thus increasing the dielectric strength of said gaseous insulation.

6. An electrical apparatus enclosed in a gas tight chamber, said electrical apparatus having spaced apart conducting means which are at different electric potentials during use of said apparatus, gaseous insulation comprised at least in part of an electronegative gas within said conducting means, and means for introducing negative ions in said gaseous insulation in addition to ions that are normally present therein during use of said apparatus, said last mentioned means comprising a nuclear radiation emitting source disposed in the circulation system of said gaseous insulation, thus increasing the dielectric strength of said gaseous insulation.,

7. An electrical apparatus enclosed in a gas tight chamber, said electrical apparatus having spaced apart conducting means which are at diierent electric potentials during use of said apparatus, gaseous insulation comprised at least in part of an electronegative gas within said conducting means, and an alpha particle emitting radiation source disposed in the circulation system of said gaseous insulation, thus increasing the dielectric strength of said gaseous insulation.

References Cited in the tile of this patent UNITED STATES PATENTS 1,157,344 Thomson Oct. 19, 1915 1,515,293 Whitehead Nov. 11, 19,24 2,128,408 Grenier Aug. 30, 1938 2,221,671 Cooper Nov. 12, 1940 2,644,026 Grenier June 30, 1953 FOREIGN PATENTS 584,405 France Nov. 20, 1924 OTHER REFERENCES Dielectric Breakdown of Sulphur Hexauoride (Works), Elec. Engr. Mag., July 1953, page 624.

Timbie and Bush: Principles of Electrical Engineering, published by John Wiley and Sons, copyright 1922; page 420 relied upon. 

